Analysis: Raised Rear Wishbones

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For many years the shape and position of the cars suspension elements have been an important factor in the cars aerodynamics. For 2013, almost every team have taken the same approach pioneered by Red Bull in 2012, by raising the rear lower wishbone. In doing this the teams have also oversized the wishbone’s cross section to enclose the driveshaft. It transpires that there are two gains from this practice, primarily improving flow over the diffuser and secondarily reducing the aerodynamic effect of the spinning driveshaft.

Rear Suspension layout

Conventional Rear suspension is formed of wishbones, a driveshaft and a trackrod.

Conventional Rear suspension is formed of wishbones, a driveshaft and a trackrod.

On all F1 cars the rear suspension layout uses effectively the same elements; there are two wishbones, one upper and one lower. Then there’s the track rod and the driveshaft. For the purposes of this discussion the use of either pull or pushrod rear suspension is irrelevant.
To obtain the ideal rear geometry the wishbones are more conventionally placed than they are at the front of the car. Rear suspension movement is far greater at the rear of the car than at the front, thus the geometry is more important. Teams will have longer wishbones due to the narrower gearbox (compared the front of the monocoque) and often employ spherical bearings, rather than flexures due to the greater angular movement of the joints. Lastly anti-squat geometry is employed to prevent the rear sitting down under acceleration.

The lower wishbone is mounted very close to the diffuser

The lower wishbone is mounted very close to the diffuser

The wishbones are spaced apart vertical to reduce the loads on the wishbones. Thus the Rear Top Wish Bone (RTWB) is mounted near the top of the gearbox case, while the Rear Lower Wish Bone (RLWB) tends to be mounted lower. Its rear leg mounting to the cover for the gearbox cross shaft, which is positioned about 140mm above the floor of the car. In this position, the lower wishbone is very close to the upper surface of the diffuser, which is 125mm above the floor. As teams are driving ever more flow over the diffuser, to activate the trailing flap or gurney to increase the pressure difference at the diffusers trailing edge.

With the top wishbone removed, the exposed driveshaft and trackrod can be seen

With the top wishbone removed, the exposed driveshaft and trackrod can be seen

Additionally the driveshaft is exposed; rules prohibit fairings specifically used for covering the driveshaft. As the driveshaft spins in the open air stream, there is an unwanted aerodynamic disruption, known as the Magnus effect.

Raised rear wishbones.

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Over recent years teams have been raised the rear wishbones for aerodynamic effect, often the RTWB is raised to position itself in front of the beam wing for a slight cascade effect, but until Red Bull’s recent design the lower wishbone remained mounted quite low on the gearbox.
Red Bull’s RB8 2012 design raised the RLWB higher up inline with the driveshaft. The rear leg of the wishbone gained a far larger cross section and this structural part of the wishbone enclosed the driveshaft and rear track rod. This approach is legal as it’s the structural wishbone covering the driveshaft and not a aerodynamic fairing. In conjunction with this, the RTWB was raised as well. This raising of the upper element was required to maintain the vertical space between the wishbones to reduce the load on the elements.

Mclaren's 2013 Raise wishbone set up, note the upper wishbone mount being inboard and above of the wheel rim

Mclaren’s 2013 Raise wishbone set up, note the upper wishbone mount being inboard and above of the wheel rim

In raising the RTWB its mounting to the upright now conflicted with the space available inside the rear wheel. Thus the outer mounting had to be moved inboard and upwards to maintain a reasonable geometry. This explains why we see the uprights extending into area normally reserved for brake ducts. Rules limit the area allowed for the upright to extend inboard and upwards of the front wheel. Team such as McLaren are now also shaping these areas into aerofoil sections for further aerodynamic benefit.

With the raised wishbones, the space above the diffuser is apparent as is the top wishbone mounting being above the wheel rim

With the raised wishbones, the space above the diffuser is apparent as is the top wishbone mounting being above the wheel rim

The aerodynamic benefit of the raised wishbone set up, is the RLWB is now spaced further from the diffuser and thus greater flow can reach the trailing edge of the diffuser.

With the top wishbone removed the oversized lower wishbone can be seen to enclose the driveshaft and trackrod

With the top wishbone removed the oversized lower wishbone can be seen to enclose the driveshaft and trackrod

The method of fairing-in the driveshaft and track rod helps reduce the blockage, further aiding flow over the diffuser while I am told that the reduction in turbulence from the Magnus effect on the driveshaft is a useful but secondary benefit.